Additive manufacturing as a technology refers to forming a solid object based on a model, as defined in standards ISO 17296-1 and ASTM 2792-12. Additive manufacturing is also known as augmented manufacturing, three dimensional printing, or 3D printing.
Thermoplastic material, typically polymer or composite material comprising matrix polymer, undergoes a transition from rigid state into a softer melt state, when heated to a temperature higher than the glass transition temperature or melt temperature of the material. Additive manufacturing by extrusion denotes a method comprising deposition of flowing thermoplastic material through a nozzle on a platform in a predefined manner to obtain a product according to a model. Hereafter, such a three-dimensional product is also referred to as a ‘3D printed product’. Additive manufacturing by extrusion differs from conventional extrusion and moulding methods, wherein a single shot of material is processed continuously into a specific shape. Typically, a mould or counter-pressure may be used to support the melt or semi-solid material, before it solidifies. An additive manufacturing by extrusion is performed without a mould. Hence, thermoplastic material used in additive manufacturing may require different characteristics than those used in conventional extrusion or moulding methods.
Material may be supplied to the additive manufacturing system in different forms, such as in a filament, powder, or granulate form. Typically, the additive manufacturing system has been configured to receive solid material in a specific form, such as in the form of a filament. Many extrusion methods, such as fused deposition modelling and fused filament fabrication, use material in a filament form. The shape and thermo-mechanical characteristics of the supplied material have an effect for the suitability of the material for additive manufacturing. Filament material pulled from a holder, also denoted as a spool, is a convenient way of providing material for an additive manufacturing system working on extrusion principle.
A filament for an additive manufacturing by extrusion may be formed of composite material comprising thermoplastic polymer as matrix material. Polylactic acid, hereafter abbreviated as PLA, may be used as matrix material in additive manufacturing by extrusion. PLA may be provided with rheological properties suitable for additive manufacturing by extrusion. PLA typically has good adhesion to a heated platform, such as glass bed, which is used in an extrusion-type additive manufacturing system to receive the melt.
One challenge of PLA in additive manufacturing by extrusion is, that the HDT and Tg of the polymer is quite low, typically around 60° C. or even less. PLA has high stiffness in temperatures below the glass transition temperature. Therefore, a filament manufactured of PLA typically is relatively brittle and has a hard, glasslike surface. Especially when kept under tension for a prolonged time, such a filament may easily break and cause interruptions into the feeding of the filament to the additive manufacturing system. In particular, when the filament feeding speed is increased, a higher amount of force may be applied on the filament, thereby increasing the risk of feeding interruptions.
The processing temperature of PLA is typically quite high. The processing temperature of PLA may be in the range of 180 to 195° C. or even higher. A high difference between the processing temperature and the Tg lead to slow cooling of melt formed from PLA. While semi-crystalline grades have a faster crystallization speed than amorphous grades of PLA, it is problematic that the PLA material in melt state may not hold the extruded shape sufficiently, before it has solidified. PLA has a relatively low melt viscosity, which may lead to messy print result in a situation where the printed material in the melt state does not stay in the extruded shape, but continues to flow and may collapse. This is particularly problematic when the dimensions of the nozzle increase and larger amounts of material are dispensed at a time.
Another aspect of additive manufacturing is, that the properties of the 3D printed product may differ from a conventionally produced extrusion product. A 3D printed product typically comprises multiple layers and adjacent rows of printed material. Depending of the printing speed and printing path, the mechanical properties of the formed 3D printed product may vary. When two adjacent rows or layers have been printed such that a subsequent row or layer is deposited as a melt either on top of or next to a solidified material layer, an interface is formed between the adhered layers. In such a situation, the material in melt state may not stop flowing in time from the nozzle. A material having low melt viscosity may be difficult to operate. This is the case in particular when the printing operation requires the production of discontinuous shapes, wherein the additive manufacturing system should leave empty space or gaps between deposited material portions or layers.
From the perspective of filament feeding, the composite material should be flexible and withstand tension. On the other hand, while being deposited from the additive manufacturing system, the composite material in melt form should allow precise portioning of the melt. It may therefore be difficult in additive manufacturing by extrusion to deposit a portion of the melt, such that each deposited portion of the composite melt may obtain a cross-dimensional shape corresponding to the dimensions of the nozzle hole.
Semi-crystalline polymers are difficult in additive manufacturing by extrusion, since semi-crystalline polymer chains cause the polymer material to shrink during cooling. Without a supporting mould, the shrinkage may lead to a 3D printed product not having sufficient dimensional precision for the end application purpose.
Hence, the characteristics of the material supplied to the additive manufacturing system may need to be considered also from the perspective of the additive manufacturing process and the formed 3D printed product.
Consequently, as described above, the properties of the material supplied to the additive manufacturing system has an effect both to the manufacturing method and the formed 3D printed product. Due to the problems described above, it may be difficult to obtain a three dimensional composite product by additive manufacturing, which would be sufficiently accurate reproduction of the model, in particular when the printing speed is increased.